About 200 people held signs and chanted Saturday outside a Walmart where a young black man with an air rifle was fatally shot by white police.

The rally was the latest effort by people wanting to shed more light on the 5 August shooting of John Crawford III inside the store in the Dayton suburb of Beavercreek. Police responded after a caller said a man was waving a gun; they say Crawford refused orders to put down an air rifle.

An attorney representing Crawford’s family says Walmart surveillance video of the shooting shows it was unjustified.

Signs and chants at the rally urged: “Release the tape.” Organisers said the Reverend Al Sharpton planned to come to the area in a few weeks’ time, to speak about the shooting.

A special grand jury will meet on 22 September to consider charges in the case. Ohio attorney general Mike DeWine said releasing the surveillance video could raise issues of a tainted jury pool if the case results in a trial.

“I think that it is playing with dynamite, frankly, to release that tape at this point,” DeWine told the Dayton Daily News. “And I think the dynamite simply is that it blows up and you can’t get a fair trial. That’s what we worry about.”

Crawford’s parents want a federal investigation and have met with US attorney Carter Stewart. Family attorney Michael Wright said Stewart assured them on Thursday that the Justice Department was monitoring and assisting Ohio’s investigation and would independently determine the merits of any potential federal prosecution.

A spokesman confirmed that federal authorities are monitoring the situation.

Another rally was planned for Sunday outside the Walmart, by people who say they want to show their support for police.